Bone homeostasis is maintained by the balanced functions of bone-forming osteoblasts (OBs) and bone- resorbing osteoclasts (OCs). Excessive OC activity can cause pathogenic bone loss, so it is important to understand the molecular signaling and genetic programs controlling the commitment, maturation, and resorption phases of OC development. Current bone loss treatments target early OC commitment and/or later viability. Long-term use of these treatments may cause compromised bone strength, possibly due to inhibition of coupled bone formation, which requires positive interplay between OCs and OBs. A better treatment strategy may be to target late-stage OC biology in favor of early OC differentiation, and in fact some recent clinical trials have successfully employed targeted inhibition of resorption without observed diminution of either OC numbers or coupled bone formation. Therefore, better understanding the regulatory factors and mechanisms of OC maturation may be extremely useful for providing better therapeutic targets. In particular, a growing number of cell adhesion molecules are now implicated in bone homeostasis and they are good potential targets for clinical intervention. Therefore, we designed a screening protocol -- described in this proposal -- to identify genes dually associated with OC maturation and cell adhesion. We now identify IgSF11, which has no previously reported function in bone, as being required specifically for progression from the commitment to the mature stages of osteoclast differentiation. We have generated and employed IGSF11-/- mice to preliminarily show that IGSF11-/- OC maturation in vitro is defective, demonstrating a potential genetic link to further pursue OC maturation-related therapeutic strategies. We therefore propose the following specific aims: 1. Investigate the effect of IgSF11 deficiency on osteoclast development and function. To begin to define the requirement for IGSF11 in OC biology per se, we will first assess early IGSF11-/- OC commitment ex vivo, and then interrogate IGSF11-/- OC maturation through gene expression and cell biologic approaches ex vivo. To examine the effects of IGSF11 deficiency on bone homeostasis, OC function and maturation in vivo, we will subject IGSF11-/- bones and bone sections obtained under normal, OVX, PTH-treated, or inflammatory conditions to high-resolution micro-computed tomography and histomorphometry, as well as TRAP staining. 2. Determine mechanisms of IgSF11 function in the context of osteoclast biology. We will determine whether IgSF11 is required for adhesion between cells during OC maturation by mixing IgSF11-expressing and IgSF11- deficient OC cultures. The effects of OC IgSF11 deficiency on expression and function of known OC- associated cell adhesion factors will be assayed. Via mutational analysis, regulation of homo- and hetero-typic interactions and cell signaling by specific IgSF11 domains retrovirally expressed in OCs will be analyzed. These experiments will address not only whether IgSF11 is required for OC cell adhesion, but also the specificity of IgSF11-mediated adhesion and potential signaling mechanisms in the context of OC maturation.